(1) Field of the Invention
The present invention relates to contact lens materials, more specifically to those having excellent oxygen permeability and water wettability, capable of being worn for extended period, and being substantially free of water absorption so that they are easy to handle.
(2) Description of the Prior Art
Contact lenses essentially made of, for example, polymethyl methacrylate have conventionally been in practical use. Polymethyl methacrylate, which is a main ingredient of the contact lenses, is highly advantageous in its excellent optical properties and durability, but it is still disadvantageous in that it cannot be worn continuously for extended period from the viewpoint of corneal physiology, due to its poor oxygen permeability.
In order to avoid the problems of the above-mentioned so-called hard type contact lenses, water-absorbable contact lenses essentially made of poly 2-hydroxyethyl methacrylate were proposed and are in practical use. Such so-called soft type contact lenses have good water affinity so that they can be somewhat comfortably worn, but they do not have sufficient oxygen permeability, making it impossible to wear for a long time. Further, because of their tendency of absorbing water, they are inferior to the hard type contact lenses because they are difficult to handle.
Contact lenses of high water content made essentially of N-vinyl pyrrolidone have been proposed for extended duration wearing. These contact lenses have good water affinity and oxygen permeability, but they have extremely poor mechanical strength due to their high water content, so that they are less durable and extremely difficult to handle.
Further, to improve the above contact lenses essentially made of polymethyl methacrylate, poly 2-hydroxyethyl methacrylate or poly N-vinyl pyrrolidone, those made of silicone rubbers or resins have been proposed. These materials, however, need a surface treatment for imparting water affinity thereto because they are hydrophobic, and the contact lenses thus surface-treated do not have satisfactory durability. As shown in U.S. Pat. Nos. 3808178, 4120570, 4306042 and 4330383, contact lenses made of modified silicone resins have been proposed, but they are not yet satisfactory.